1. Field of the Invention
The invention relates to a solenoid valve assembly, and more particularly, relates to a method and apparatus for calibrating an assembled variable force solenoid.
2. Description of the Related Art
A typical solenoid comprises a coil of wire that produces a magnetic field when electrical current flows through it. When the magnetic flux passes through a stationary pole piece and a movable armature, the armature is drawn toward pole piece to actuate and control an attached mechanical device through a push rod. A spring element within the solenoid resists the force generated by the magnetic field, and biases the push rod towards its original position. An air gap will typically exist between the pole piece and armature, thereby reducing the magnetic flux during operation of the solenoid.
In a solenoid valve assembly, a solenoid transforms electrical inputs into hydraulic outputs such as the hydraulic pressure or flow that controls various hydraulic devices of a vehicle. For example, in an automatic transmission controller, an input current regulates the hydraulic output pressure, which may be either directly or inversely proportional to the amount of current flowing through the coils of the solenoid. In an inversely proportional solenoid valve assembly, for example, maximum current induces minimum pressure, and minimum current induces maximum pressure. A variable force solenoid (VFS) is a solenoid valve assembly having an integral feedback mechanism.
Achieving accurate control over a VFS requires the balancing of three forces that act on the solenoid: 1) the electromagnetic force produced by the electrical current flowing through the coil; 2) the spring force resisting the electromagnetic force; and 3) the hydraulic feedback force. Improper balancing of these forces may result in undesirable changes in hydraulic output or performance of the solenoid valve. Balancing these three forces requires a properly designed variable force solenoid.
Unfortunately, even if a VFS is properly designed, difficulties remain in providing proper calibration to maintain proper operation of the solenoid. For example, in U.S. Pat. No. 5,197,507, one conventional method of calibrating a solenoid includes the insertion of a predetermined number of metallic spacers between the solenoid housing and valve body, thereby matching electromagnetic force with hydraulic operating point. However, once the assembly of the solenoid valve has been completed, adjusting the number of spacers within the air gap is difficult.
Another calibration process, disclosed in U.S. Pat. No. 4,947,893, involves adding, around a spring adjustment screw, an air gap adjustment plug. The spring adjustment screw is used to calibrate the spring pre-load and, thus, the output pressure at zero current. The air gap adjustment plug is to adjust the minimum air gap and, thus, the electromagnetic force and the resulting output pressure variation. Because of their physical nesting, the movement of the air gap adjustment screw will cause a corresponding movement in the spring adjustment screw, which necessitates adjustment iterations. Furthermore, these two screws nested together are relatively expensive and inefficient to manufacture.
What is therefore needed is a simplified method and apparatus for calibrating a variable force solenoid after assembly of the solenoid valve has been completed.
In accordance with a first aspect of the invention, a variable force solenoid is provided having a flux path including a variable size air gap disposed therein so as to permit the adjustment of the amount of magnetic flux that acts on the armature after assembly of the solenoid. A primary air gap is disposed between the magnetic pole piece and the armature. A secondary air gap is formed between the out wall of the armature and the inner wall of the solenoid housing. The solenoid may be calibrated by first energizing the solenoid coils and determining a corresponding desired hydraulic pressure, measuring the actual hydraulic pressure, and adjusting the magnetic flux until the desired output pressure is achieved.
These as well as other features and characteristics of the present invention will be apparent from the description which follows. In the detailed description below, preferred embodiments of the invention will be described with reference to the accompanying drawings. These embodiments do not represent the full scope of the invention. Rather the invention may be employed in other embodiments, and reference should therefore be made to the claims herein for interpreting the breadth of the invention.